An ambulatory medical device, such as an implantable medical device (IMD), can be configured for implant in a subject, such as a patient. An IMD can be configured to be coupled to a patient's heart such as via one or more implantable leads. Such an IMD can obtain diagnostic information or generate therapy to be provided to the patient, such as via the coupled implantable lead. Examples of such devices can include cardiac rhythm management (CRM) devices including one or more of implantable pacemakers, implantable cardioverter-defibrillators (ICDs), cardiac resynchronization therapy devices (CRTs), neural stimulators, or one or more other devices. Such devices can include one or more electrodes coupled, such as via the implantable lead, to circuitry located on or within the IMD. Such circuitry can be configured to monitor electrical activity, such as to obtain information indicative of electrical activity of the heart. In one configuration, IMDs have a header that is coupled to a container that houses much of the electronics of the IMD.